Winter Poisoning

"In ancient times, women would worship peach blossom tree and make offerings to the tree's spirit to be blessed with a child. However, the spirit of tree is a malevolent one." Shen Zi Yu told Yi Sun.
"It always takes back what is promised to it even when the pact-maker is gone"

A spirit of the tree blesses a lady of Jiang family with a healthy male child. However, a pact is made, and after Yi Sun's mother who died tried protecting her son, it will take back what rightfully belongs to it. The one who's born out of the promise must return the debt as it will take back its lover back to death.